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[Pages:48]Lists

Chris Piech and Mehran Sahami CS106A, Stanford University

Piech + Sahami, CS106A, Stanford University

Housekeeping I

? Assignment #2 due today ? Assignment #3 goes out today (Due on Wed., Oct. 14)

? Can do Part 1 after today's class ? Practice with lists (which will be on diagnostic)

? Can study for diagnostic and get part of assignment done! ? Can do Part 2 after this coming Monday's class

Piech + Sahami, CS106A, Stanford University

Housekeeping II

? Diagnostic assessment on Wed., Oct. 7

? Takes place during class time ? Covers through today's material (i.e., lists are fair game) ? Please download BlueBook software before the exam ? There is a practice diagnostic (and instructions) on class website ? If you have OAE accommodations or are in time zone (outside the

Americas) that requires rescheduling, and haven't heard from Juliette, please email her

Piech + Sahami, CS106A, Stanford University

Global Variables: Bad Style

# Constant ? visible to all functions NUM_DAYS_IN_WEEK = 7

# Global variable ? visible to all functions balance = 0

Different variables with the same name!

def main():

Super confusing!

balance = int(input("Initial balance: "))

while True:

amount = int(input("Deposit (0 to quit): "))

if amount == 0:

break deposit(amount)

? Also, really BAD style

? So bad, that Python won't even let you do

it unless you basically add a command

def deposit(amount): balance += amount

that says "I want to have bad style"

? I'm not going to show you that command in Python

? But, if you know it already, DON'T use it!

? We're in polite company

Using Parameters: Good Style

Don't want using your toaster to impact your refrigerator!

def main(): balance = int(input("Initial balance: ")) while True: amount = int(input("Deposit (0 to quit): ")) if amount == 0: break balance = deposit(balance, amount)

def deposit(balance, amount): balance += amount return balance

Encapsulation Principle: Data used by a function should be a parameter or encapsulated in function

The Python Console

? Can run Python interactively using the "console"

? In PyCharm click "Python Console" tab at bottom of window ? In Terminal, run Python (e.g., typing "py" or "python3" or

"python", depending on your platform) to get console

? Console has prompt: >>>

? Can type and execute Python statements (and see results) ? Example:

>>> x = 5 >>> x 5

? Easy way to try things out to answer questions you may have ? Use exit() to leave console

Piech + Sahami, CS106A, Stanford University

Let's Take the Console Out For a Spin...

Piech + Sahami, CS106A, Stanford University

And Then There Were None

? The term None is used in Python to describe "no value"

? For example, it is the value you would get from a function that doesn't return anything

? WHAT?! ? Example:

>>> x = print("hi") >>> print(x) None

? Comparing anything to None (except None) is False

? Why does None exist?

? Denotes when the suitcase for a variable has "nothing" in it

Piech + Sahami, CS106A, Stanford University

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